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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2021

SDSS-IV MaNGA: 3D spin alignment of spiral and S0 galaxies

ABSTRACT We investigate the 3D spin alignment of galaxies with respect to the large-scale filaments using the MaNGA survey. The cosmic web is reconstructed from the Sloan Digital Sky Survey using disperse and the 3D spins of MaNGA galaxies are estimated using the thin disc approximation with integral field spectroscopy kinematics. Late-type spiral galaxies are found to have their spins parallel to the closest filament’s axis. The alignment signal is found to be dominated by low-mass spirals. Spins of S0-type galaxies tend to be oriented preferentially in perpendicular direction with respect to the filament’s axis. This orthogonal orientation is found to be dominated by S0s that show a notable misalignment between their kinematic components of stellar and ionized gas velocity fields and/or by low-mass S0s with lower rotation support compared to their high-mass counterparts. Qualitatively similar results are obtained when splitting galaxies based on the degree of ordered stellar rotation, such that galaxies with high spin magnitude have their spin aligned, and those with low spin magnitude in perpendicular direction to the filaments. In the context of conditional tidal torque theory, these findings suggest that galaxies’ spins retain memory of their larger scale environment. In agreement with measurements from hydrodynamical cosmological simulations, the measured signal at low redshift is weak, yet statistically significant. The dependence of the spin-filament orientation of galaxies on their stellar mass, morphology, and kinematics highlights the importance of sample selection to detect the signal.

Kraljic, Katarina↗
DOE OSTI2021

Kiloparsec-scale AGN outflows and feedback in merger-free galaxies

ABSTRACT Recent observations and simulations have challenged the long-held paradigm that mergers are the dominant mechanism driving the growth of both galaxies and supermassive black holes (SMBH), in favour of non-merger (secular) processes. In this pilot study of merger-free SMBH and galaxy growth, we use Keck Cosmic Web Imager spectral observations to examine four low-redshift (0.043 < z < 0.073) disc-dominated ‘bulgeless’ galaxies hosting luminous active galactic nucleus (AGN), assumed to be merger-free. We detect blueshifted broadened [O iii] emission from outflows in all four sources, which the $\mathrm{[O\, \small {III}]}$/$\rm {H}\beta$ ratios reveal are ionized by the AGN. We calculate outflow rates in the range $0.12{-}0.7~\rm {M}_{\odot }~\rm {yr}^{-1}$, with velocities of $675{-}1710~\rm {km}~\rm {s}^{-1}$, large radial extents of $0.6{-}2.4~\rm {kpc}$, and SMBH accretion rates of $0.02{-}0.07~\rm {M}_{\odot }~\rm {yr}^{-1}$. We find that the outflow rates, kinematics, and energy injection rates are typical of the wider population of low-redshift AGN, and have velocities exceeding the galaxy escape velocity by a factor of ∼30, suggesting that these outflows will have a substantial impact through AGN feedback. Therefore, if both merger-driven and non-merger-driven SMBH growth lead to co-evolution, this suggests that co-evolution is regulated by feedback in both scenarios. Simulations find that bars and spiral arms can drive inflows to galactic centers at rates an order of magnitude larger than the combined SMBH accretion and outflow rates of our four targets. This work therefore provides further evidence that non-merger processes are sufficient to fuel SMBH growth and AGN outflows in disc galaxies.

Smethurst, R. J. (ORCID:0000000164177196)↗
DOE OSTI2021

The galaxy–halo connection of emission-line galaxies in IllustrisTNG

ABSTRACT We employ the hydrodynamical simulation IllustrisTNG-300-1 to explore the halo occupation distribution (HOD) and environmental dependence of luminous star-forming emission-line galaxies (ELGs) at z ∼ 1. Such galaxies are key targets for current and upcoming cosmological surveys. We select model galaxies through cuts in colour–colour space allowing for a direct comparison with the Extended Baryon Oscillation Spectroscopic Survey and the Dark Energy Spectroscopic Instrument (DESI) surveys and then compare them with galaxies selected based on specific star formation rate (sSFR) and stellar mass. We demonstrate that the ELG populations are twice more likely to reside in lower density regions (sheets) compared with the mass-selected populations and twice less likely to occupy the densest regions of the cosmic web (knots). We also show that the colour-selected and sSFR-selected ELGs exhibit very similar occupation and clustering statistics, finding that the agreement is best for lower redshifts. In contrast with the mass-selected sample, the occupation of haloes by a central ELG peaks at ∼20 per cent. We furthermore explore the dependence of the HOD and the autocorrelation on environment, noticing that at fixed halo mass, galaxies in high-density regions cluster about 10 times more strongly than low-density ones. This result suggests that we should model carefully the galaxy–halo relation and implement assembly bias effects into our models (estimated at ∼4 per cent of the clustering of the DESI colour-selected sample at z = 0.8). Finally, we apply a simple mock recipe to recover the clustering on large scales (r ≳ 1 Mpc h−1) to within 1 per cent by augmenting the HOD model with an environment dependence, demonstrating the power of adopting flexible population models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

FORECASTOR – II. Simulating galaxy surveys with the Cosmological Advanced Survey Telescope for Optical and UV Research

The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a planned flagship space telescope, covering the blue-optical and UV part of the spectrum. Here, we introduce the CASTOR image simulator, a python GalSim package-based script capable of generating mock CASTOR images from an input catalogue. We generate example images from the CASTOR Wide, Deep, and Ultra-Deep surveys using simulated lightcones from the Santa Cruz semi-analytic model. We make predictions for the performance of these surveys by comparing galaxies that are extracted from each image using Source Extractor to the input catalogue. We find that the Wide, Deep, and Ultra-Deep surveys will be 75 per cent complete for point sources down to $\sim 27$, 29, and 30 mag, respectively, in the UV, u, and g filters, with the UV-split and u-split filters reaching a shallower depth. With a large area of $\sim 2200$ deg$^2$, the Wide survey will detect hundreds of millions of galaxies out to $z\sim 4$, mostly with $M_\ast \gtrsim 10^{9}\,{\rm M}_{\odot }$. The Ultra-Deep survey will probe to $z\sim 5$, detecting galaxies with $M_\ast \gtrsim 10^{7}{\rm M}_{\odot }$. These galaxy samples will enable precision measurements of the distribution of star formation in the cosmic web, connecting the growth of stellar mass to the assembly of dark matter haloes over two thirds of the history of the Universe, and other core goals of CASTOR’s legacy surveys. These image simulations and the tools developed to generate them will be a vital planning tool to estimate CASTOR’s performance and iterate the telescope and survey designs prior to launch.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

PAC in DESI. I. Galaxy stellar mass function into the 10 6 M ⊙ frontier

The Photometric objects Around Cosmic webs (PAC) method integrates cosmological photometric and spectroscopic surveys, offering valuable insights into galaxy formation. PAC measures the excess surface density of photometric objects, $\bar{n}_2w_{{\rm {p}}}$, with specific physical properties around spectroscopic tracers. In this study, we improve the PAC method to make it more rigorous and eliminate the need for redshift bins. We apply the enhanced PAC method to the DESI Y1 BGS Bright spectroscopic sample and the deep Dark Energy Camera Legacy Survey (DECaLS) photometric sample, obtaining $\bar{n}_2w_{{\mathrm {p}}}$ measurements across the complete stellar mass range, from $10^{5.3}$ to $10^{11.5}\,{\rm M}_{\odot }$ for blue galaxies, and from $10^{6.3}$ to $10^{11.9}\,{\rm M}_{\odot }$ for red galaxies. We combine $\bar{n}_2w_{{\rm {p}}}$ with $w_{{\rm {p}}}$ measurements from the BGS sample, which is not necessarily complete in stellar mass. Assuming that galaxy bias is primarily determined by stellar mass and colour, we derive the galaxy stellar mass functions (GSMFs) down to $10^{5.3}\,{\rm M}_{\odot }$ for blue galaxies and $10^{6.3}\,{\rm M}_{\odot }$ for red galaxies, while also setting lower limits for smaller masses. The blue and red GSMFs are well described by single and double Schechter functions, respectively, with low-mass end slopes of $\alpha _{\rm {blue}}=-1.54^{+0.02}_{-0.02}$ and $\alpha _{\rm {red}}=-2.50^{+0.08}_{-0.08}$, resulting in the dominance of red galaxies below $10^{7.6}\,{\rm M}_{\odot }$. Stage-IV cosmological photometric surveys, capable of reaching 2–3 mag deeper than DECaLS, present an opportunity to explore the entire galaxy population in the local universe with PAC. This advancement allows us to address critical questions regarding the nature of dark matter, the physics of reionization, and the formation of dwarf galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

PAC in DESI. II. Galaxy-halo connection into the $10^{6}{\rm M}_{\odot}$ frontier

Understanding dwarf galaxy formation is crucial for testing dark matter models and reionization physics. However, constructing stellar-mass complete spectroscopic samples at low masses is increasingly difficult, and the potential existence of a local void complicates studies in an average environment. The Photometric object Around Cosmic webs (PAC) method, which combines deep photometric and spectroscopic data to measure the excess surface density $\bar{n}_2w_{\rm{p}}(r_{\rm{p}})$ of photometric objects around spectroscopic tracers, offers a promising path forward. We model 349 $\bar{n}_2w_{\rm{p}}(r_{\rm{p}})$ measurements from DESI Y1 BGS and DECaLS, reaching $M_*=10^{6.4}\,{\rm M}_{\odot}$, using a stellar mass-halo mass relation (SHMR)-based subhalo abundance matching framework applied to two high-resolution $N$-body simulations from the Jiutian suite. The resulting SHMR is constrained down to $M_{\rm h}\simeq10^{8.0}\,h^{-1}{\rm M}_{\odot}$, revealing a clear upturn at $\sim10^{10.0}\,h^{-1}{\rm M}_{\odot}$ toward lower masses, indicating rising star-formation efficiency (SFE) in small haloes. This feature persists under extensions of the model that allow mass-dependent scatter, reionization-induced suppression of the halo occupation fraction, galaxy assembly bias, and alternative cosmologies. Combining with the results from Paper I, we find that central red galaxies dominate the low-mass regime. Our results motivate a hypothesis in which SFE is significantly higher than previously thought prior to reionization, enabling relatively massive galaxies to form in small haloes. These systems are subsequently quenched by the UV background, producing the central red dwarf galaxies observed. Finally, we obtain $3σ$ and $5σ$ upper mass bounds of $10^{8.80}\,h^{-1}{\rm M}_{\odot}$ and $10^{10.24}\,h^{-1}{\rm M}_{\odot}$ on the smallest haloes required to exist.

Xu, Kun [Pennsylvania U.; Durham U., ICC; Tsung-Da↗
DOE OSTI2019

Probing Galaxy assembly bias in BOSS galaxies using void probabilities

ABSTRACT We measure the void probability function (VPF) of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). The VPF provides complementary information to standard two-point statistics in that it is sensitive to galaxy bias in the most extreme underdensities in the cosmic web. Thus, the VPF is ideal for testing whether halo occupation of galaxies depends on large-scale density, an effect known as galaxy assembly bias. We find that standard HOD model – one parameterized by halo mass only – fit only to the two-point function, accurately predicts the VPF. Additionally, for HOD models where density dependence is explicitly incorporated, the best-fitting models fit to the combination of the correlation function and the VPF have zero density dependence. Thus, galaxy assembly bias is not a strong source of systematic uncertainty when modelling the clustering of massive galaxies.

Walsh, Kilian↗
DOE OSTI2018

BAM: bias assignment method to generate mock catalogues

Combining the statistics of dark matter tracers from a high-resolution cosmological N-body simulation and the dark matter density field calculated from down-sampled initial conditions using efficient structure formation solvers, we extract the halo-bias relation on a mesh of a 3$h^{-1}$ Mpc cell side resolution as a function of properties of the dark matter density field (e.g. local density, cosmic web type), automatically including stochastic, deterministic, local and non-local components. We use this information to sample the halo density field, accounting for ignored dependencies through an iterative process. By construction, our approach reaches ~1% accuracy in the majority of the k-range up to the Nyquist frequency without systematic deviations for power spectra (about $k ~ 1h$ Mpc -1 ) using either particle mesh or Lagrangian perturbation theory based solvers. When using phase-space mapping to compensate the low resolution of the approximate gravity solvers, our method reproduces the bispectra of the reference within 10% precision studying configurations tracing the quasi-non-linear regime. BAM has the potential to become a standard technique to produce mock halo and galaxy catalogues for future galaxy surveys and cosmological studies being highly accurate, efficient and parameter free.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Thermal and Chemical Evolution of Collapsing Filaments

Intergalactic filaments form the foundation of the cosmic web that connect galaxies together, and provide an important reservoir of gas for galaxy growth and accretion. Here we present very high resolution two-dimensional simulations of the thermal and chemical evolution of such filaments, making use of a 32 species chemistry network that tracks the evolution of key molecules formed from hydrogen, oxygen, and carbon. We study the evolution of filaments over a wide range of parameters including the initial density, initial temperature, strength of the dissociating UV background, and metallicity. In low-redshift, Z ≈ 0.1Z ⊙ filaments, the evolution is determined completely by the initial cooling time. If this is sufficiently short, the center of the filament always collapses to form dense, cold core containing a substantial fraction of molecules. In high-redshift, Z = 10 -3 Z ⊙ filaments, the collapse proceeds much more slowly. This is due mostly to the lower initial temperatures, which leads to a much more modest increase in density before the atomic cooling limit is reached, making subsequent molecular cooling much less efficient. Finally, we study how the gravitational potential from a nearby dwarf galaxy affects the collapse of the filament and compare this to NGC 5253, a nearby starbusting dwarf galaxy thought to be fueled by the accretion of filament gas. In contrast to our fiducial case, a substantial density peak forms at the center of the potential. This peak evolves faster than the rest of the filament due to the increased rate at which chemical species form and cooling occur. We find that we achieve similar accretion rates as NGC 5253, but our two-dimensional simulations do not recover the formation of the giant molecular clouds that are seen in radio observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Fast radio bursts as probes of magnetic fields in the intergalactic medium

We examine the proposal that the dispersion measures (DMs) and Faraday rotation measures (RMs) of extragalactic linearly polarized fast radio bursts (FRBs) can be used to probe the intergalactic magnetic field (IGMF) in filaments of galaxies. The DM through the cosmic web is dominated by contributions from the warm-hot intergalactic medium (WHIM) in filaments and from the gas in voids. On the other hand, RM is induced mostly by the hot medium in galaxy clusters, and only a fraction of it is produced in the WHIM. We show that if one excludes FRBs whose sightlines pass through galaxy clusters, the line of sight (LOS) strength of the IGMF in filaments, B{sub ||}, is approximately C(〈1+z〉/f{sub DM})(RM/DM), where C is a known constant. Here, the redshift of the FRB is not required to be known; f {sub DM} is the fraction of total DM due to the WHIM, while 〈1+z〉 is the redshift of interevening gas weighted by the WHIM gas density, both of which can be evaluated for a given cosmology model solely from the DM of an FRB. Using data on structure formation simulations and a model IGMF, we show that C(〈1+z〉/f{sub DM})(RM/DM) closely reproduces the density-weighted LOS strength of the IGMF in filaments of the large-scale structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Alignments of dark matter halos with large-scale tidal fields: mass and redshift dependence

Large-scale tidal fields estimated directly from the distribution of dark matter halos are used to investigate how halo shapes and spin vectors are aligned with the cosmic web. The major, intermediate, and minor axes of halos are aligned with the corresponding tidal axes, and halo spin axes tend to be parallel with the intermediate axes and perpendicular to the major axes of the tidal field. The strengths of these alignments generally increase with halo mass and redshift, but the dependence is only on the peak height, ν≡δ{sub c}/σ(M{sub h},z). The scaling relations of the alignment strengths with the value of ν indicate that the alignment strengths remain roughly constant when the structures within which the halos reside are still in a quasi-linear regime, but decreases as nonlinear evolution becomes more important. We also calculate the alignments in projection so that our results can be compared directly with observations. Finally, we investigate the alignments of tidal tensors on large scales, and use the results to understand alignments of halo pairs separated at various distances. Our results suggest that the coherent structure of the tidal field is the underlying reason for the alignments of halos and galaxies seen in numerical simulations and in observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Galaxy groups in the 2MASS redshift survey

A galaxy group catalog is constructed from the 2MASS Redshift Survey (2MRS) with the use of a halo-based group finder. The halo mass associated with a group is estimated using a “GAP” method based on the luminosity of the central galaxy and its gap with other member galaxies. Tests using mock samples show that this method is reliable, particularly for poor systems containing only a few members. On average, 80% of all the groups have completeness >0.8, and about 65% of the groups have zero contamination. Halo masses are estimated with a typical uncertainty of ∼0.35 dex. The application of the group finder to the 2MRS gives 29,904 groups from a total of 43,246 galaxies at z⩽0.08, with 5286 groups having two or more members. Some basic properties of this group catalog is presented, and comparisons are made with other group catalogs in overlap regions. With a depth to z∼0.08 and uniformly covering about 91% of the whole sky, this group catalog provides a useful database to study galaxies in the local cosmic web, and to reconstruct the mass distribution in the local universe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

The Neutral Gas Properties of Extremely Isolated Early-type Galaxies III

We report on the neutral hydrogen gas content (21 cm emission) of eight extremely isolated early-type galaxies (IEGs) using the Green Bank Telescope. Emission is detected in seven of the eight objects. This paper is the third in a series that collectively present new H i observations for 20 IEGs. Among the 14 H i detections in our observations, eight exhibit a Gaussian-like H i line profile shape, four are double-peaked, one is triple-peaked, and another has a plateaued rectangular shape. Five additional IEGs observed in previous surveys were added to our analysis, bringing the total number of IEGs with H i observations to 25. Of these objects, emission is detected in 19 (76%). The 25 IEGs in our combined study have gas masses that are systematically larger than their luminosity-matched comparison galaxy counterparts. The IEGs presented here follow a trend of increasing gas-richness with bluer B - V colors. This correlation is also observed in a comparison sample drawn from the literature composed of loose group and field early-type galaxies. Two IEGs, KIG 164 and KIG 870, exhibit properties highly anomalous for spheroidal systems: luminous (M {sub B} = -20.5, -20.1) and blue (B - V = 0.47, 0.48), respectively, with substantial neutral gas, M {sub H i} = 4.1 and 5.5 × 10{sup 9} M {sub &odot;}. Other IEG systems may represent early-type galaxies continuing to assemble via quiescent H i accretion from the cosmic web or relaxed merged systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Most Metal-poor Stars in the Magellanic Clouds Are r-process Enhanced*

The chemical abundances of a galaxy’s metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most massive of the Milky Way’s satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Way’s less massive satellites, little is known about the Magellanic Clouds’ metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey and archival data to target nine LMC and four SMC giants for high-resolution Magellan/MIKE spectroscopy. These nine LMC giants with -2.4 ≲ [Fe/H] ≲ -1.5 and four SMC giants with -2.6 ≲ [Fe/H] ≲ -2.0 are the most metal-poor stars in the Magellanic Clouds yet subject to a comprehensive abundance analysis. While we find that at constant metallicity these stars are similar to Milky Way stars in their α, light, and iron-peak elemental abundances, both the LMC and SMC are enhanced relative to the Milky Way in the r-process element europium. These abundance offsets are highly significant, equivalent to 3.9σ for the LMC, 2.7σ for the SMC, and 5.0σ for the complete Magellanic Cloud sample. We propose that the r-process enhancement of the Magellanic Clouds’ metal-poor stellar population is a result of the Magellanic Clouds’ isolated chemical evolution and long history of accretion from the cosmic web combined with r-process nucleosynthesis on a timescale longer than the core-collapse supernova timescale but shorter than or comparable to the thermonuclear (i.e., Type Ia) supernova timescale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

KINEMATICS OF EXTREMELY METAL-POOR GALAXIES: EVIDENCE FOR STELLAR FEEDBACK

The extremely metal-poor (XMP) galaxies analyzed in a previous paper have large star-forming regions with a metallicity lower than the rest of the galaxy. Such a chemical inhomogeneity reveals the external origin of the metal-poor gas fueling star formation, possibly indicating accretion from the cosmic web. This paper studies the kinematic properties of the ionized gas in these galaxies. Most XMPs have a rotation velocity around a few tens of km s{sup −1}. The star-forming regions appear to move coherently. The velocity is constant within each region, and the velocity dispersion sometimes increases within the star-forming clump toward the galaxy midpoint, suggesting inspiral motion toward the galaxy center. Other regions present a local maximum in velocity dispersion at their center, suggesting a moderate global expansion. The H α line wings show a number of faint emission features with amplitudes around a few per cent of the main H α component, and wavelength shifts between 100 and 400 km s{sup −1}. The components are often paired, so that red and blue emission features with similar amplitudes and shifts appear simultaneously. Assuming the faint emission to be produced by expanding shell-like structures, the inferred mass loading factor (mass loss rate divided by star formation rate) exceeds 10. Since the expansion velocity far exceeds the rotational and turbulent velocities, the gas may eventually escape from the galaxy disk. The observed motions involve energies consistent with the kinetic energy released by individual core-collapse supernovae. Alternative explanations for the faint emission have been considered and discarded.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Radial Distribution of Mono-metallicity Populations in the Galactic Disk as Evidence for Two-phase Disk Formation

Recent determinations of the radial distributions of mono-metallicity populations (MMPs, i.e., stars in narrow bins in [Fe/H] within wider [α/Fe] ranges) by the SDSS-III/APOGEE DR12 survey cast doubts on the classical thin- and thick-disk dichotomy. The analysis of these observations led to the non-[α/Fe] enhanced populations splitting into MMPs with different surface densities according to their [Fe/H]. By contrast, [α/Fe] enhanced (i.e., old) populations show a homogeneous behavior. We analyze these results in the wider context of disk formation within non-isolated halos embedded in the Cosmic Web, resulting in a two-phase mass assembly. By performing hydrodynamical simulations in the context of the ΛCDM model, we have found that the two phases of halo mass assembly (an early fast phase, followed by a slow phase with low mass-assembly rates) are very relevant to determine the radial structure of MMP distributions, while radial mixing only plays a secondary role, depending on the coeval dynamical and/or destabilizing events. Indeed, while the frequent dynamical violent events occuring at high redshift remove metallicity gradients and imply efficient stellar mixing, the relatively quiescent dynamics after the transition keeps [Fe/H] gaseous gradients and prevents newly formed stars from suffering strong radial mixing. By linking the two-component disk concept with the two-phase halo mass-assembly scenario, our results set halo virialization (the event marking the transition from the fast to the slow phases) as the separating event that marks periods that are characterized by different physical conditions under which thick- and thin-disk stars were born.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Cold Filamentary Accretion and the Formation of Metal-poor Globular Clusters and Halo Stars

We propose that cold filamentary accretion in massive galaxies at high redshifts can lead to the formation of star-forming clumps in the halos of these galaxies without dark matter substructure. In certain cases, these clumps can be the birthplaces of metal-poor globular clusters (MP GCs). Using cosmological simulations, we show that narrow streams of dense gas feeding massive galaxies from the cosmic web can fragment, producing star-forming clumps. We then derive an analytical model for the properties of streams as a function of halo mass and redshift, and assess when these are gravitationally unstable, when this can lead to collapse and star formation in the halo, and when it may result in the formation of MP GCs. For stream metallicities $\gtrsim 0.01{Z}_{\odot }$, this is likely to occur at $z\gt 4.5$. At $z\sim 6$, the collapsing clouds have masses of $\sim (5\mbox{--}10)\times {10}^{7}\,{M}_{\odot }$, and the average stream pressure is $\sim {10}^{6}\,{\mathrm{cm}}^{-3}\,{\rm{K}}$. The conditions for GC formation are met in the extremely turbulent "eyewall" at $\sim 0.3{R}_{{\rm{v}}}$, where counter-rotating streams can collide, driving very large densities. Our scenario can account for the observed kinematics and spatial distribution of MP GCs, the correlation between their mass and metallicity, and the mass ratio between the GC system and the host halo. For MW-mass halos, we infer that $\sim 30 \% $ of MP GCs could have formed in this way, with the remainder likely accreted in mergers. Our predictions for GC formation along circumgalactic filaments at high redshift are testable with JWST.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

The Location and Environments of Neutron Star Mergers in an Evolving Universe

The simultaneous detection of gravitational and electromagnetic waves from a binary neutron star merger has both solidified the link between neutron star mergers and short-duration gamma-ray bursts (GRBs), and demonstrated the ability of astronomers to follow-up the gravitational wave detection to place constraints on the ejecta from these mergers, as well as the nature of the GRB engine and its surroundings. As the sensitivity of aLIGO and VIRGO increases, it is likely that a growing number of such detections will occur in the next few years, leading to a sufficiently large number of events to constrain the populations of these GRB events. While long-duration GRBs originate from massive stars and thus are located near their stellar nurseries, binary neutron stars may merge on much longer timescales, and thus may have had time to migrate appreciably. The strength and character of the electromagnetic afterglow emission of binary neutron star mergers is a sensitive function of the circum-merger environment. Though the explosion sites of short GRBs have been explored in the literature, the question has yet to be fully addressed in its cosmological context. We present cosmological simulations following the evolution of a galaxy cluster, including star formation combined with binary population synthesis models, to self-consistently track the locations and environmental gas densities of compact binary merger sites throughout the cosmic web. In conclusion, we present probability distributions for densities as a function of redshift and discuss model sensitivity to population synthesis model assumptions.

79 ASTRONOMY AND ASTROPHYSICS↗